Near-Earth asteroid Bennu was long thought by scientists to have surface like a sandy beach, abundant in fine sand and pebbles, and to also have large swaths of fine-grained material smaller than a few centimetres called fine regolith.

However, NASA's OSIRIS-REx mission, which arrived at Bennu in late 2018, saw a surface covered in boulders. The mysterious lack of fine regolith became even more surprising when mission scientists observed evidence of processes potentially capable of grinding boulders into fine regolith, the US space agency said.

To understand better, a team of scientists at the University of Arizona conducted a research using machine learning and surface temperature data.

This image shows a view of asteroid Bennu's surface in a region near the equator. It was taken by the PolyCam camera on NASA's OSIRIS-REx spacecraft on March 21, 2019 from a distance of 2.2 miles (3.5 km). (Credits: NASA/Goddard/University of Arizona)NASA/Goddard/University of Arizona

The findings, published in the journal Nature, showed that Bennu's highly porous rocks are responsible for the surface's surprising lack of fine regolith.

"When the first images of Bennu came in, we noted some areas where the resolution was not high enough to see whether there were small rocks or fine regolith. We started using our machine learning approach to distinguish fine regolith from rocks using thermal emission (infrared) data," said Saverio Cambioni from the varsity's Lunar and Planetary Laboratory, in a statement.

The thermal emission from fine regolith is different from that of larger rocks, because the size of its particles controls the former, while the latter is controlled by rock porosity. The team first built a library of thermal emissions associated with fine regolith mixed in different proportions with rocks of various porosity.

Next, they used machine-learning techniques to teach a computer how to "connect the dots" between the examples, Cambioni said. They analysed 122 areas on the surface of Bennu, that were observed both during the day and the night.

Cambioni also found that the fine regolith was not randomly distributed on Bennu. Instead, it was up to several tens of percent in those very few areas where rocks are non-porous, and systematically lower where rocks have higher porosity, which is most of the surface.

Asteroid Bennu

The team concluded that very little fine regolith is produced from Bennu's highly porous rocks because these are compressed rather than fragmented by meteoroid impacts. Like a sponge, the voids within rocks cushion the blow from incoming meteoroids. These findings are also in agreement with laboratory experiments from other research groups, NASA said.

In addition, the team showed that cracking caused by the heating and cooling of Bennu's rocks as the asteroid rotates through day and night proceeds more slowly in porous rocks than in denser rocks, further frustrating the production of fine regolith.

The OSIRIS-REx spacecraft is due to reach Earth on September 24, 2023, after orbiting the Sun twice.